Remote Phonon Scattering in Si and Ge with SiO2 and HfO2 Insulators: Does the Electron Mobility Determine Short Channel Performance?

Abstract

We have computed the low-field electron mobility in inversion layers for single-gate metal–oxide–semiconductor-field-effect-transistor (MOSFET) structures with Si and Ge substrates and high-κ gate dielectrics. Scattering with bulk phonons, surface roughness, and surface optical phonons has been included. The mobility has been studied as a function of electron sheet density, gate dielectric, temperature, equivalent dielectric thickness and for polycrystalline Si and metallic gates. We have also performed full-band Monte Carlo simulations for Si and Ge MOSFETs with gate lengths of 60, 30, and 15 nm, employing SiO2 and HfO2 as gate dielectrics. Our results show that high-κ interface optical modes affect Si and Ge roughly by the same amount, the Ge mobility in HfO2 system still remaining larger than in the Si–SiO2 system. However, the low-field mobility does not entirely determine device performance, measured as transconductance. The transconductance degradation caused by high-κ phonons is much smaller than the mobility degradation, yielding a negligible negative impact, especially in Ge devices, shorter devices, and in the saturation region of operation.